The present invention relates generally to the field of electroplating baths. In particular, the present invention relates to the analysis and control of products in copper electroplating baths.
Electroplating is a complex process involving multiple ingredient, in a plating bath. It is important that the concentration of several of the ingredients be kept with in close tolerances in order to obtain a high quality deposit. In some cases, chemical analysis of individual solution constituents can be made regularly (such as pH measurement for acid con tent), and additions made as required. However, other addition agents such as brighteners, leveling agents, suppressants, etc., together with impurities, cannot be individually analyzed on an economical or timely basis by a commercial plating shop. Their operating concentration is low and their quantitative analysis is complicated and subject to error.
A known method for controlling such ingredients in an electroplating bath is to make regular additions of particular ingredients based upon empirical rules established by experience. However, depletion of particular ingredients is not always constant with time or with bath use. Consequently, the concentration of the ingredients is not actually known ;and the level in the bath eventually diminishes or increases to a level where it is out of the acceptable range tolerance. If the additive content goes too far out of range, the quality of the metal deposit suffers and the deposit may be dull in appearance and/or brittle or powdery in structure. Other possible consequences include low throwing power and/or plating folds with bad leveling.
Another known method for plating bath control is to plate articles or samples and visually evaluate the plating quality to determine if the bath is performing satisfactorily. In standard Hull Cell and xe2x80x9cBone Patternxe2x80x9d tests, a specially shaped test specimen is plated a d then evaluated to determine the quality of the deposit along with shape. This is a time consuming test which gives at best a rough approximation of the concentration of the constituents of the bath.
The electroplating of through-hole interconnections in the manufacture of multilayer printed circuit boards is an example of the use of an electroplating metal where high quality plating is required. It is known that the concentration of the organic additives, such as brighteners and levelers, within the plating solution must be maintained in low concentration (typically less than 100 parts per million parts of solutionxe2x80x94ppm) in order to obtain acceptable deposits on printed circuit boards. This must be done to maintain proper mechanical properties for resistance to thermal stresses encountered during manufacture and use and to assure the proper thickness of the deposit in the through-holes and leveling. The concentration of the organic additive agents fluctuates because of oxidation at the anode. Reduction at the cathode, and chemical degradation. When the additive level is insufficient, deposits are burned and powdery in appearance whereas excessive addition agents induce brittleness and non-uniform deposition. Hull cell tests, Bone Pattern tests, and Pencil tests, combined with periodic additions of fresh additives, were the methods used to maintain a control concentration of the additive until recently. These methods were unreliable and circuit board quality suffered as a consequence of these unreliable methods.
Conventional electroplating bath analytical tools measure the total electrochemical activity of the plating bath. Such total activity includes the total organic additives present including any additive breakdown products. For example, U.S. Pat. No. 4,917,774 (Fisher) discloses a method in which a small amount of metal is electrodeposited onto an inert electrode such as platinum or gold under controlled conditions of electrode potential and mass transport in the solution. The amount of metal deposited is determined by integrating the current passed during redissolution or xe2x80x9cstrippingxe2x80x9d of the deposited metal from the surface as the electrode potential proceeds through a cycle including plating and stripping. The quantity of metal deposited, and subsequently redissolved, is related to the concentration of additives affecting the rate of deposition. Such methods are used to analyze only the particular additives present in the plating baths.
U.S. Pat. No. 5,223,118 (Sonnenberg et al.) discloses a method for determining the quantity of brighteners and levelers present in an electroplating bath for the plating of printed wiring board substrates. In this method, the determination of both brightener and leveler is accomplished in one step.
In the electroplating of electronic devices having small features, such as wafers used in the manufacture of integrated circuits or semiconductors, the control of additive concentration in the plating bath is very important. The concentrations of certain additive in electroplating baths, particularly copper plating baths, must be maintained within certain limits in order to obtain superfilling, or bottom-up fill, during plating of wafers. Many of the known compounds that provide superfill undergo redox reactions at both the electrode surfaces and in solution. Known reactions that occur in the electroplating bath include reduction of the additive at the cathode, i.e. wafer, during plating, oxidation of the additive at the copper anode during, plating, reduction of the additive at the copper anode during idling, and formation of highly electroactive copper complexes from reduction byproducts. Such reactions can have deleterious effects on the ability of the plating bath to perform at acceptable levels. Thus there is a need for a method of determining the quantity additive breakdown products in such electroplating baths.
Heretofore, no methods have been proposed that will accurately determine the amount of additive breakdown products in an electroplating bath, particularly in the presence of unreacted additive compound. There is thus a need for analytical methods for determining and controlling the amount of additive and additive breakdown product present in copper electroplating baths, particularly in wafer electroplating.
It has been surprisingly found that the methods of the present invention provide a means for analyzing the concentrations of certain additive breakdown products as well as a means for controlling the concentrations of such components in the plating bath. The present invention allows for the measurement of additive breakdown product in the parts per billion range in presence of the additive and directly in the bath.
The present invention provides a method for determining the quantity of additive breakdown products in an electroplating bath including the steps of: a) obtaining a plurality of plating baths where each bath has a known quantity of the additive breakdown products, but where the quantity of additive breakdown product in each bath differs from the quantity in the other baths: b) for each bath, providing a counter electrode, a cleaned working electrode and a reference electrode immersed in the bath, and equilibrating the working electrode without energy input to adsorb additive breakdown product for a period of time until the change in potential of the working electrode with time is minimal and measuring the value of the potential; c) for each bath, correlating the quantity of additive breakdown product with the value of potential obtained in step b); d) obtaining a plating bath having an unknown quantity of additive breakdown product, placing the electrodes in the bath and performing step b); and e) choosing from the correlations in step c a quantity of additive breakdown product which corresponds to the equilibrated working electrode potential for the bath with the unknown quantity of additive breakdown product.
The present invention also provides a method for controlling the presence of additive breakdown products in an electroplating bath including the steps of: a) determining the quantity of additive breakdown product as described above; and b) comparing the quantity of additive breakdown product to a preset value; provided that when the quantity of additive breakdown product exceeds the preset value, a control process is activated that reduce the quantity of additive breakdown product.
The present invention further provides a method of maintaining within wafer uniformity during the electroplating of wafers including the step of controlling the level of additive breakdown product in the electroplating bath.